Clinical experience and psychophysical research indicate that the configuration of electrical currents on each channel of a cochlear prosthesis can have great impact on recognition of speech and other signals. The goal of this research program is to elucidate mechanisms relevant to normal and prosthetic hearing that can guide development of clinical stimulation strategies. The proposed experiments will compare the quality of stimulus representation in the auditory cortex determined by various cochlear current configurations. Monopolar and other configurations that are presumed to produce diffuse neural activation will be compared with bipolar and other configurations thought to produce more focal activation. Experiments will be conducted in guinea pigs. Specific Aim 1 is to characterize the distribution of unit activity across cortical location and post-stimulus time in response to stimulation of a single cochlear channel. Multi-channel cortical recording probes will permit simultaneous unit recording at 16 cortical sites. Artificial neural-network techniques will test the accuracy with which cochlear stimulation channels can be identified on the basis of patterns of cortical activity. Stimulus current levels will be expressed relative to psychophysical thresholds assessed in individual subjects. Specific Aim 2 will test interactions between cochlear channels, using single pulses and amplitude-modulated pulse trains. Cortical phase locking to stimulus modulation will be quantified, then the influence of a simultaneous or interleaved masking pulse train presented on a second cochlear channel will be assessed. Animal psychophysical experiments in Specific Aim 3 will measure thresholds for modulation detection in the presence and absence of a masker on a second cochlear channel. Stimulus conditions will be compared that, in the cortical experiments, either minimize or maximize interaction between cochlear channels. Specific Aim 4 is to identify current configurations that are particularly suited to various histories of deafness and stimulation. The results will be used to examine the central mechanisms that underlie effects of duration of deafness and stimulation. These results will be used to examine the central mechanisms that underlie effects of duration of deafness on prosthesis function.